The stock IS adjustable though...like the 6 position, sort of. The 13 ci will require it to be a certain length though.
Yeah I'm familiar with it... in a theoretical sense. I saw it a long time ago when looking at mag fed markers and wishing. I'm fairly certain that even with it adjusted to it's shortest length a marker with anything but the shortest stub nose barrel will still be pretty long for going through a doorway. I used to have a regular M4 stock on my gun and while it worked great at close quarters with the 3" stub nose barrel, with my Flatline on or even my standard barrel it was way too long for close quarters.
I'd rather have a folding stock (or one of those MP5 style sliders) and not HAVE to switch from Flatline to Stubnose barrel if I don't have time to.